In recent years, with electric power shortage becoming serious, there have been challenges that are global rapid adoption of natural energy such as the adoption of wind power generation and solar photovoltaic power generation and the stabilization of power systems (for example, maintaining of frequency and voltage). One technique for addressing the challenges has been attracting attention and this technique is to install high-capacity storage batteries to achieve, for example, smoothing of output variations, storage of surplus power, and load leveling.
One of such high-capacity storage batteries is a redox flow battery (hereafter, sometimes referred to as a RF battery). The RF battery is charged and discharged by using the oxidation-reduction potential difference between ions contained in the positive electrode electrolyte and ions contained in the negative electrode electrolyte. FIG. 6 illustrates the principle of the operation of a RF battery 100 using vanadium ions as the positive and negative active materials. As illustrated in FIG. 6, the RF battery 100 includes a battery cell 100C that is divided into a positive electrode cell 102 and a negative electrode cell 103 by a membrane 101, which is permeable to hydrogen ions (protons). The positive electrode cell 102 contains a positive electrode 104, and is connected through ducts 108 and 110 to a positive electrode electrolyte tank 106 storing a positive electrode electrolyte. Similarly, the negative electrode cell 103 contains a negative electrode 105, and is connected through ducts 109 and 111 to a negative electrode electrolyte tank 107 storing a negative electrode electrolyte. The electrolytes stored in the tanks 106 and 107 are circulated through the cells 102 and 103 by pumps 112 and 113 during charge and discharge.
As illustrated in the lower part of FIG. 7, in general, the battery cell 100C is formed within a structure referred to as a cell stack 200. As illustrated in the upper part of FIG. 7, the cell stack 200 has a configuration in which such battery cells 100C having the positive electrode 104, the membrane 101, and the negative electrode 105 are stacked so as to be interposed between cell frames 120, which include a bipolar plate 121 integrated with a frame 122. In this configuration, each battery cell 100C is formed between bipolar plates 121 of adjacent cell frames 120. The gaps between the cell frames 120 are sealed with sealing structures 127.
In the cell stack 200, the electrolytes are passed through the battery cells 100C via liquid supply manifolds 123 and 124 and liquid drainage manifolds 125 and 126, which are formed in the frames 122. The positive electrode electrolyte is supplied through the liquid supply manifold 123, then through a groove formed in one surface (the surface illustrated as being exposed in the drawing) of the frame 122, to the positive electrode 104, which is disposed on the one surface of the bipolar plate 121. This positive electrode electrolyte is drained through a groove formed in an upper portion of the frame 122 to the liquid drainage manifold 125. Similarly, the negative electrode electrolyte is supplied through the liquid supply manifold 124, then through a groove formed in the other surface (the surface illustrated as being hidden in the drawing) of the frame 122, to the negative electrode 105, which is disposed on the other surface of the bipolar plate 121. This negative electrode electrolyte is drained through a groove formed in an upper portion of the frame 122 to the liquid drainage manifold 126.
The RF battery is charged with power supplied from, for example, a power plant via, for example, an alternating current/direct current converter; and is discharged to supply the charged power to a load via, for example, the alternating current/direct current converter. In order to obtain a larger power from the power system or to supply a larger power to the load, battery cell parts including battery cells as the main components (for example, cell stacks described above) may be electrically connected in parallel. Patent Literature 1 discloses a RF battery in which a single cell stack includes plural divided cells (hereafter, sometimes referred to as battery cell parts). This RF battery includes a switching unit that enables selection of desired battery cell parts, electrical connection of the selected battery cell parts in parallel, and charge and discharge of the selected battery cell parts. This RF battery can be operated with high energy efficiency in response to the amount of charge or discharge.